1. Technical Field
The invention relates to the field of control technology. It relates to a method and a device for controlling a compressor unit.
2. Prior Art
A turbocompressor is inherently stable during normal operation: on the basis of the inlet and outlet pressure as well as parameters of the compressor, a mass flow of a working fluid through the compressor is established. This flow, which may be considered as a volume flow or mass flow, decreases as the pressure differential increases, so that the pressures and the flow move toward a state of equilibrium. With a falling pressure differential, a stable operating range is limited by a so-called surge limit: with increasing outlet pressure, the mass flow falls to a certain minimum. After exceeding the surge limit, the mass flow flows back through the compressor. As a result, the outlet pressure falls, until the mass flow flows forward again. This cycle, known as surging, is repeated and may mechanically damage or destroy the compressor. Therefore, apart from controlling an outlet pressure or flow, it is a task of a compressor controller to avoid surging. For this purpose, as described in U.S. Pat. No. 4,807,150, usually a blowoff or return valve is opened, allowing part of the compressed working fluid to escape, or returning it to the inlet of the compressor. At the same time as the valve opens, the rotational speed of the compressor may also be varied, as disclosed in U.S. Pat. No. 5,306,116. The additional mass flow through the return valve prevents the mass flow through the compressor from going below the surge limit. Usually, the compressor and the return valve are controlled by their own closed-loop control circuits.
For safety reasons, the return valve is already opened before the surge limit is exceeded. A corresponding safety limit should be as far as possible from the surge limit. To optimize efficiency, on the other hand, the safety limit should be as close as possible to the surge limit. This requires safety precautions which increase the complexity of the control circuits.
A combination of a compressor with a return valve is referred to hereafter as a compressor unit. A compressor unit which supplies a gas turbine with gaseous fuel must satisfy high requirements concerning pressure control. For example, when there are abrupt changes in the load of the gas turbine and an associated change in gas consumption, the output pressure of the compressor must be maintained without a flame of the gas turbine being extinguished, and without lines being damaged due to excessive pressures. During normal operation, no oscillations may occur in the gas delivery. Depending on the controller concept of the gas turbine, it is also possible that a compressor unit must supply a prescribed mass flow. In the case of hydraulic systems, on the other hand, control to a prescribed volume flow is of interest.
Axial- or radial-flow compressors are equipped with adjustable rows of inlet guide vanes for the purpose of varying the flow rate. Another way of varying the flow rate uses a variable-speed drive of the compressor. Given constant inlet and outlet conditions, in both cases the mass flow is dependent on an angle of the row of inlet guide vanes and, respectively, a rotational speed. A corresponding controller for a compressor unit controls at least two manipulated variables, for example the angle of the row of inlet guide vanes and the return valve. The existing controller structures are complex and have decoupled controllers for the two manipulated variables, which usually makes a systematic controller design impossible. Instances of switching over between different operating states make the dynamics of the controller and consequently of the compressor obscure and consequently even more difficult to design and put into operation.
It is therefore the object of the invention to provide a method and a device for controlling the outlet pressure of a compressor unit which has a simple structure and makes a systematic controller design possible.
This object is achieved by a method and a device for controlling the outlet pressure of a compressor unit having the features of patent claims 1 and 7.
In the controller according to the invention for a compressor unit which has a compressor and a return valve, a characteristic variable for an overall flow to be supplied is determined, and a first setpoint value for a row of guide vanes or an inlet valve or a rotational speed of the compressor and a second setpoint value for a return valve are generated on the basis of this characteristic variable by means of static functions.
The overall flow to be supplied is preferably a mass flow, but may also be a volume flow.
In a preferred embodiment of the subject-matter of the invention, the overall flow is set in a normal operating range by variation of the first setpoint value, and when the normal operating range is left is set by variation of the second setpoint value. Advantageously, the overall flow thereby changes continuously during the transition between these operating ranges.
In a further preferred embodiment of the subject-matter of the invention, the static functions for determining the first and second setpoint values are linear.
Parameters of the static functions are advantageously adapted to an operating state of the compressor.
Further preferred embodiments emerge from the dependent patent claims.